I've received a fair number of questions on the Intel Virtualization and Software Development Forum about VT-d -- enough to justify a few words on this note in the blogging sphere.

"VT-d" stands for "Intel Virtualization Technology for Directed I/O". The relationship between VT and VT-d is that the former is an "umbrella" term referring to all Intel virtualization technologies and the latter is a particular solution within a suite of solutions under this umbrella.

The overall concept behind VT-d is hardware support for isolating and restricting device accesses to the owner of the partition managing the device.

A VMM may support various models for I/O virtualization, including emulating the device API, assigning physical I/O devices to VMs, or permitting I/O device sharing in various manners. The key problem is how to isolate device access so that one resource cannot access a device being managed by another resource.

VT-d, at the time of this writing, includes four key capabilities

1. I/O device assignment. This feature allows an administrator to assign I/O devices to VMs in any desired configuration.

Note that VT-d is not dependent on VT-x. That is, a VT-x enabled system can operate without VT-d, or without VT-d enabled or configured. You simply miss the benefits of the feature. Many people have asked about this point.